Also check out www.payloadz.com - ran by Paypal. They also have merchant services (well they have had them for a long time) but just started to market them.

If you do get your own merchant account, you might consider vetting your account so that you can reduce your chargebacks some. Downloadable goods do have a higher risk of chargebacks - unless of course, you can turn off the registration somehow. That is usually what I recommend to my clients - set something up in your software that connects to your server for verification. This way, if they do a chargeback stating the software does not work like it should, you can always disable their account